1. Simple procedures involving an affinity chromatography step were devised to purify adenylyltransferase and uridylyltransferase from crude extracts of E. coli strain which has been engineered to overproduce them. 2. The mechanisms by which glutamine, Alpha-ketoglutarate, and nucleotide monophosphate affect the uridylylation-deuridylylation reactions were studied in detail. In addition, kinetic parameters for these two opposing reactions were determined. 3. The variation in the concentration of uridylyltransferase and adenylyltransferase influences the rate of synthesis of GS in vivo. Studies with strains harboring a multicopy plasmid indicate that the elevated synthesis of the converter enzymes causes the enhanced GS synthesis in E. coli when grown on a nitrogen excess medium. The parallel relationship between the converter enzymes and GS seems to be due to the fact that the concentration of unbound PII, which is required to repress the synthesis of GS, is dependent on the concentration of converter enzymes. 4. Many proteins, represented by yeast glutamine synthetase, E. coli adenylyltransferase, rabbit muscle pyruvate kinase, and E. coli glutamine synthetase are degraded in the presence of DTT and catalytic amounts of iron salts. This process requires oxygen, and the role of DTT and iron can be relaced by ascorbate and copper, respectively. Experimental data suggest that reactive oxygen species, likely hydroxyl radicals, generated locally around iron bound at the specific sites of enzymes are responsible for the degradation. In view of this finding, caution must be taken to prevent this type of oxidative modification when an enzyme is exposed to DTT because many biochemicals are contaminated with metal salts in sufficient quantities for catalyzing the formation of hydroxyl radicals.